1. Field of the Invention
Ethylene-unsaturated polycarboxylic compound copolymers, especially ethylene-maleic anhydride copolymers, are employed in coating compositions having a multiplicity of uses. These copolymers, usually graft copolymers, are economically important in that they used as coatings on a whole host of substrates. This is not surprising in that ethylene polymers provide excellent coatability on a great number of substrates. The adhesivity imparted by the unsaturated polycarboxylic compound grafted onto the ethylene polymer provides an excellent bond in holding the uniform coating onto the substrate on which is applied.
A major problem in this art, however, has been the inability in the past to graft a high enough concentration of unsaturated polycarboxylic compound onto the ethylene polymer to provide the desirable level of adhesion and emulsifiability, two properties essential to a good coating. In the prior art, this problem was addressed by grafting an adhesive monomer, i.e., a dicarboxylic unsaturated acid, onto an ethylene polymer in the presence of an organic peroxide which initiates a free radical reaction wherein the unsaturated compound is grafted onto the ethylene polymer. Although this method effectively grafts the unsaturated polycarboxylic compound onto the ethylene polymer, it also catalyzes a secondary, undesirable reaction. That is, the polymeric radicals generated by the decomposition of the peroxide combine to form polymeric crosslinks. As those skilled in the art are aware, the degree of crosslinkage manifests itself in undesirably high gel content. Thus, a dilemma in this art has been the development of ethylene-unsaturated polycarboxylic compound graft copolymers which although possessing the requisite degrees of adhesivity and emulsifiability are also characterized by insufficient degrees of thermoplasticity. That is, undesirable crosslinkage, as manifested by high gel content, in the prior art has been the price paid to obtain the essential level of unsaturated polycarboxylic compound grafted onto an ethylene polymer to provide a desirable coating product.
2. Background of the Prior Art
U.S. Pat. No. 3,236,917 to Natta et al. describes the graft copolymerization of an ethylene-propylene copolymer with maleic anhydride in the presence of an organic peroxide which initiates a graft copolymerization reaction. This reaction deliberately results in a crosslinked polymer. Indeed, crosslinkage is promoted by the presence of a basic compound, such as zinc oxide, which acts a crosslinking catalyst.
U.S. Pat. No. 3,484,403 to Brunson et al. teaches a hot melt coating and adhesive blend of a polyamide and a polyolefin which therebefore could not be blended. The advance in the art of the '403 patent lies in initially modifying the polyolefin by reacting it with an unsaturated dicarboxylic acid or its anhydride. This initial reaction, resulting in the formation of a graft copolymer, involves contact between a polyolefin with an unsaturated dicarboxylic acid or anhydride. This reaction is conducted at elevated temperature in the range of between about 200.degree. C. and 400.degree. C. and in absence of oxygen and a catalyst, such as an organic peroxide.
That the graft copolymer of the '403 patent requires blending with a polyamide to produce a desirable coating or adhesive establishes that the degree of grafting in the absence of an organic peroxide is alone insufficient to provide an acceptable coating or adhesive.
U.S. Pat. No. 3,644,248 to Luijk discloses the addition of maleic anhydride to polyisoprene in the presence of mastication or an organic peroxide. As in the case of U.S. Pat. No. 3,236,917, this reaction is deliberately crosslinked. Indeed, the above reaction is followed by crosslinkage of the elastomer by contact with a divalent Group II or IV metal oxide catalyst.
U.S. Pat. No. 3,884,882 to Caywood, Jr., in an attempt to control crosslinkage, sets forth a process for the formation of a thermoplastic elastomer by the thermal addition of maleic anhydride to elastomeric copolymers of ethylene, at least one C.sub.3 to C.sub.6 alpha-olefin and at least one nonconjugated diene, typically, the copolymer of ethylene, propylene and at least one non-conjugated diene, commonly referred to as EPDM. The gel content of the thus formed thermoplastic elastomer is less than 5% by weight, based on the total weight of the polymer. This product is characterized by the absence of a free radical generator such as an organic peroxide. However, this product is thereafter cured. Obviously, this product is not used as a coating or adhesive.
U.S. Pat. No. 3,642,722 to Knowles et al. provides a continuous process for producing modified polyolefins, preferably polyethylene, in which a low molecular weight crystallizable polyolefin, formed by thermally degrading a high molecular weight polyolefin, is reacted with an ethylenically unsaturated polycarboxylic compound, preferably maleic anhydride, under plug-type flow conditions in an agitated, multi-staged reactor. In an illustrated embodiment the polyolefin and the polycarboxylic compound are introduced at the bottom of the reaction zone and the desirable graft polymer product is recovered from the top of that zone. Preferably, the reaction rate of this process is accelerated by the use of an organic peroxide, preferably ditertiary butyl peroxide.
U.S. Pat. No. 4,028,436 to Bogan et al. teaches a process for modifying polyethylene waxes to make them emulsifiable. In this process low viscosity polyethylene, prepared by thermal degradation of conventional high molecular weight ethylene homopolymer, is reacted with an organic unsaturated polycarboxylic acid in the presence of a free radical source in the substantial absence of oxygen. The unsaturated polycarboxylic acid may be replaced with an acid anhydride or an acid ester derived therefrom. The absence of oxygen eliminates an important problem associated with such modified polyethylene waxes, their characteristic yellow or orange color.
U.S. Pat. No. 4,376,855 to Ames sets forth a process directed to the modification or grafting of low viscosity polyethylene or ethylene-alpha-olefin copolymer waxes to provide improved physical properties. In the process of the '855 patent a low viscosity ethylene polymer wax is reacted with an unsaturated polycarboxylic compound in the presence of an excessive amount of an organic peroxide, added incrementally during the reaction wherein the weight ratio of unsaturated polycarboxylic component, which is preferably maleic anhydride, to organic peroxide is in the range of between about 1:1 to less than about 5:1. This process is said to substantially eliminate crosslinkage, albeit without the necessity of excluding oxygen from the reaction.
U.S. Pat. No. 4,358,564 to Ames provides a similar process to that disclosed in the Ames '855 patent. In this process a low viscosity ethylene polymer, characterized by less than 0.1% unsaturation, is reacted with an unsaturated polycarboxylic component in the presence of a free radical source, usually an organic peroxide, in a concentration of about 1% to about 10% by weight, based on the weight of the unmodified low viscosity ethylene polymer, and at least one viscosity stabilizer. A particularly preferred class of viscosity stabilizers is recited to be alkyl thiols.
The above discussion of the prior art establishes that although the prior art has proposed a multiplicity of processes to produce graft copolymers of an ethylene polymer and an unsaturated polycarboxylic component, none of them produce the desirable combination of low crosslinkage combined with high carboxylic functionality necessary to provide excellent coating and adhesive properties. Therefore, there is a continuing need in the art for new and better processes to provide this desired modified ethylene polymer.